Hybrid circuits should be encapsulated to insure resistor durability in humid atmospheres. Furthermore, manufacturers prefer glass encapsulation to protect the conductor metals from long term corrosion.
The encapsulant system must exhibit several features which are difficult to achieve together. It must form a bubble-free seal at low enough firing temperature and prevent shift of the underlying resistors. If the glass flows too much, it will diffuse into the resistor and shift the value upward. If it does not flow enough, it will not seal. The organic vehicle necessary for screen printing must burn out at this low temperature. Thus an ideal encapsulant should screen print smoothly and rapidly with a vehicle which is decomposable at a low enough temperature to allow the glass to flow sufficiently to form a seal, but not so much as to shift the resistor.
Various glasses having low glass transition temperature (Tg) have been used extensively as encapsulants for electronic circuits. These glasses usually have had a high Temperature Coefficient of Expansion (TCE) which, unless it is carefully matched to the adjacent circuit layers, can set up substantial mechanical stresses which can lead to system failures.
An encapsulant, among its other functions, provides protection from the environments to the underlying electronic circuit. To fullfill this function the encapsulant should have sufficient durability to survive the environments encountered in the production and the daily use of the electronic circuits. Most low softening point glasses (referred to here as "low melting glasses") have poor durability in acids and bases and their durability tends to degrade as the glass transition temperature (Tg) becomes lower. Although the majority of electronic circuits are not expected to be used in very acidic or basic environments, some are exposed to water and basic or acidic environments during the production. The final stage in some fabrication processes involves an additional encapsulation by an organic polymer, e.g., an epoxy. Some epoxy resins contain an amine which can exert basic environment in humid atmosphere. Therefore, durabilities in boiling water and basic solutions (triethanol amino TEA, in water to simulate amines in epoxy) are detailed here.
To combat this problem, a glass has been suggested by Asahi Glass KK in JPA 52/154825, which is a crystallizable zinc-lead-borate type glass that undergoes crystallization when it is fired at 540.degree.-560.degree. C. and produces a crystallized overlay having a low TCE. Although the glass forms a dense overlay when fired at 540.degree. C., the layer tends to be porous because of insufficient flow of the vitreous phase and excessive crystallization. It is, of course, desirable to be able to fire at a temperature in the 510.degree.-560.degree. C. range in order to avoid interaction of the glass with the underlying circuit components during the firing cycle. Therefore, there remains a real need for an encapsulating glass which (1) can be fired in the 510.degree.-560.degree. C. range to form a dense overlay, and (2) will form a dense overlay having good encapsulating properties.